In the machining of oil country tubular goods a large dedicated threading machine is normally used to form threads for pipe and connectors. The dedicated machine usually comprises a die head with three tool blocks spaced one hundred twenty degrees apart for holding turning tools that remove excess metal stock from the part prior to the cutting of threads. The die head additionally comprises three tool blocks spaced intermediate the turning tool blocks for holding three unique thread-cutting chasers. This machine can produce a threaded end at extremely high speeds utilizing a single machine pass operation. However, dedicated threading machines of this type usually cannot be used to produce threads that require close tolerances and enhanced surface finish. In order to meet the demands of the industry for critical applications set forth by the American Petroleum Institute with regard to tighter thread tolerances on 8-round and buttress connections the use of slower computer numerial controlled (CNC) machines has been required. The CNC machine utilizes a single carbide insert and multiple machine passes to produce these threaded parts. These multiple pass methods enhance thread quality by lowering the machine cutting forces imposed on the machine and the part. Even with the use of CNC machines thread burrs on the starting and ending threads are common and must be removed with additional carbide cutting steps or in most cases must be removed by hand dressing.
U.S. Pat. No. 3,082,446 discloses a die head for a rotary spindle thread cutting machine employing three high-speed carbide thread-cutting chasers for cutting tapered threads. U.S. Pat. No. 3,812,548 shows a similar rotary spindle thread cutting machine with a die head having three thread cutting chasers. U.S. Pat. Nos. 3,776,655 and 3,629,887 both disclose a set of three multitooth chasers arranged to perform a succession of central roughing cuts and one finish cut both having chip segments of approximately trapezoidal cross sections. The three chasers are each of unique profile and are said to be free from any portions that can cause flanking cuts that result in lateral flanges on the resultant chips. A die head for a rotary spindle machine is shown with positions for three chasers for cutting the threads.
U.S. Pat. No. 3,645,638 describes a series of substantially identical individual single tooth thread cutting tools that are rotated concurrently about the axis of a pipe and applied in cutting relation in succession at the same starting point by movement radially in a time delay relation to each other. The cutting tools are retracted from the pipe in succession in the same order in which they were engaged with the pipe in the same time delay relation to each other. A die head with eight identical cutting tools is shown.
U.S. Pat. No. 2,267,506 discloses a diestock for cutting threads on pipe with four multitooth chasers spaced ninety degrees from each other. U.S. Pat. No. 2,556,742 also discloses a die head with four thread chasers arranged at spaced locations circumferentially arranged ninety degrees from each other. Neither of these references teach or suggest that the chasers may each be of unique profile.
Other references of interest are U.S. Pat. Nos. 5,002,440; 4,117,563; 1,647,037; 1,459,240; 1,418,323; 1,076,188; 861,682; and 828,808.